Specific regions

Circle of Willis

A2-segment Part of anterior cerebral artery distal to the anterior communicating artery.

P1-segmentPart of the posterior cerebral artery proximal to the posterior communicating artery.
The posterior communicating artery is between the carotid bifurcation and the posterior cerebral artery)

P2-segmentPart of the posterior cerebral artery distal to the posterior communicating artery

M1-segment
Horizontal part of the middle cerebral artery which gives rise to the lateral lenticulostriate arteries which supply most of the basal ganglia.
The M2-segment is the part in the sylvian fissure and the M3-segment is the cortical segment.

Cisterna ambiensAlso called ambient cistern is a cistern of the subarachnoid space between the posterior end of the corpus callosum and the superior surface of the cerebellum.
It is sometimes defined as including the quadrigerminal cistern.

On the left a coronal view of the segments of the middle cerebral artery.

Horizontal M1-segment
gives rise to the lateral lenticulostriate arteries which supply part of head and body of caudate, globus pallidus, putamen and the posterior limb of the internal capsule.
Notice that the medial lenticulostriate arteries arise from the A1-segment of the anterior cerebral artery.